In the following description, publications referenced herein are identified with bracketed numbers (e.g., [1]) which refer to the list of references identified in the List of References following the description. Optical high-voltage sensors often rely on the electro-optic effect (Pockels effect) in crystalline materials such as Bi4Ge3O12 (BGO) [1]. An applied voltage introduces a differential optical phase shift between two orthogonal, linearly polarized light waves propagating through the crystal. This phase shift is proportional to the voltage. At the end of the crystal, the light waves commonly interfere at a polarizer. The resulting light intensity serves as a measure for the phase shift and thus the voltage.
U.S. Pat. No. 4,904,931 [2] and U.S. Pat. No. 6,252,388 [3] disclose a sensor in which the full line voltage (up to several 100 kV) is applied over the length of a single BGO crystal. The crystal length is generally between 100 mm and 250 mm. An advantage is that the sensor signal corresponds to the true voltage, that is, the line integral of the electric field along the crystal. However, the electric field strengths at the crystal are very high. In order to obtain sufficient dielectric strength, the crystal is mounted in a hollow high-voltage insulator made of fiber-reinforced epoxy filled with SF6 gas under pressure for electric insulation. The electrodes at the crystal ends are designed so that the field along the crystal is reasonably homogeneous. The insulator diameter is sufficiently large to keep the field strength in the air outside the insulator below critical limits. The field strength generally decreases with increasing radial distance from the crystal.
U.S. Pat. No. 6,252,388 [4] discloses a voltage sensor which uses several small electro-optical crystals mounted at selected positions along the longitudinal axis of a hollow high-voltage insulator. The crystals measure the electric fields at their locations. The sum of these local field measurements serves as an approximation of the voltage applied to the insulator. Here, the field strengths at a given voltage are significantly lower than with the design of [2] and insulation with nitrogen at atmospheric pressure is sufficient. However, since the sensor does not measure the line integral of the field but derives the signal from the field strengths at a few selected points between ground and high voltage, extra measures (permittivity-shielding) to stabilize the electric field distribution are necessary to avoid excessive approximation errors [5].
A drawback of the above concepts is the requirement of an expensive high-voltage insulator of large size. The outer dimensions are similar to the ones of corresponding conventional inductive voltage transformers or capacitive voltage dividers. Thus, the attractiveness of such optical sensors is limited.
Ref. [6] discloses a sensor in which the voltage is partitioned among several quartz crystals, each with a length of 150 mm, for example. Here, the piezo-electric deformation of the crystals under the applied voltage is transmitted to an optical fiber, which carries at least two different light modes. The light waves travelling through the fiber experience a differential optical phase shift in proportion to the voltage. The ends of each crystal are again equipped with electrodes that provide a relatively homogenous field distribution at the crystals. The electrodes of adjacent crystals are interconnected with electric conductors. The voltage partitioning reduces the electric field strengths compared to a solution with a single crystal and thus makes it possible to mount the crystals in a relatively slender high-voltage insulator of relatively low cost. The hollow volume of the insulator is filled with soft polyurethane. A drawback is that relatively large corona rings are required in order to ensure that the voltage drops at the individual crystals are of comparable magnitude. Furthermore, enhanced electric field strengths occur particularly at the outer surface of the insulator near the positions of the individual electrodes: The peak fields must be kept below the breakdown field of air and therefore prevent still smaller insulator diameter.
Ref. [7] describes an electro-optical voltage sensor of the type as in [2, 3], but with an electro-optic crystal embedded in silicone. A hollow high-voltage insulator of large size and SF6-gas insulation is thus avoided. As in [6] the voltage may be partitioned among several crystals.
Other prior art is a concept as known from high-voltage bushings. There is often a need in high-voltage systems to pass high-voltage conductors through or near by other conductive parts which are at ground potential (for example at power transformers). For this purpose the high-voltage conductor is contained within a feed-through insulator. The insulator contains several layers of metal foil concentric with the high-voltage conductor and insulated from each other. By appropriately choosing the length of the individual cylinders of metal foils, the distribution of the electric field within and near the bushing can be controlled in such a way that a relatively homogeneous voltage drop from high-voltage to ground potential occurs along the outer surface of the bushing [8, 9, 10].